The closest platform that is a somewhat known group of people coming together in a semi-private space seems to be Discord.

You can have as large or as small a community you want, you can have known people, unknown people.

Mastodon comes close as well, but the effort to start a discord community is so much smaller compared to running a Mastodon instance.

Wouldn't this also extend to Matrix/Mumble/XMPP/TeamSpeak then?